1. Field of Invention
This invention relates to a new and improved resonant piston compressor having an improved piston stroke control and to a heat pump apparatus utilizing such compressor.
More particularly, the invention relates to a resonant free-piston variable stroke compressor having improved means for controlling the stroke of the resonant piston compressor (RPC). In RPC's, the spring-mass system of the resonant free-piston is forced into vibration by the armature of a linear reciprocating motor with the armature being connected to and comprising part of the spring-mass system of the resonant free-piston, the entire subassembly being referred to hereinafter as the plunger. The plunger generally comprises at least one piston-armature adapted to be driven within a selectively valved working cylinder by the linear motor in synchronism with the frequency of an AC power source used to drive the linear motor. Enclosed volumes of gas acting as resilient gas spring means coact with the plunger assembly to form a mechanical resonant system having a resonant frequency centered on the frequency of the AC power source. In the present invention, improved means are provided for controlling the power output of the linear motor and plunger assembly by varying the resonant frequency of the mechanical resonant system to increase the resonant frequency by increasing the stiffness of the resilient gas spring means in response to increasing piston stroke, and decreasing the resonant frequency by decreasing the stiffness of the resilient gas springs in response to decreasing piston stroke.
2. Background of Invention
A controlled stroke, resonant free-piston compressor employing an electrodynamic linear motor is described and claimed in the U.S. Pat. No. 3,937,600 issued Feb. 10, 1976 to Harlan V. White--Inventor and entitled, "Controlled Stroke Electrodynamic Linear Compressor"--assigned to Mechanical Technology Incorporated. An improved control system for use in conjunction with the present invention is described in copending U.S. application Ser. No. 160,305, Richard A. Dorman--Inventor, filed June 17, 1980 concurrently with this application, now U.S. Pat. No. 4,291,258 and entitled, "Control System for Resonant Free-Piston Variable Stroke Compressor for Load-Following Electric Heat Pumps and the Like," assigned to Mechanical Technology Incorporated. A further improved controlled stroke, resonant free-piston compressor with which the present invention and control system described in copending U.S. application Ser. No. 160,305, can be used as described in U.S. Pat. No. 4,067,667 for a "Control Stroke Electrodynamic Oscillating Motor Compressor," Harlan V. White--Inventor also assigned to Mechanical Technology Incorporated of Latham, New York. The disclosures of each of these prior art reference United States patents and copending application, are hereby incorporated in their entirety into this application. RPC heat pumps, for the purpose of this disclosure, are defined as load-following (modulating) heat pumps where the words "load-following" or "modulating" mean that the output heating or cooling capacity of the heat pump is varied in a substantially continuous manner so that its output capacity matches the actual instantaneous heating/cooling load requirement. For example, if the load is a single-family residence where the heating or cooling requirements vary as a function of outdoor weather conditions, the heating or cooling capacity of an RPC heat pump automatically will be smoothly varied (modulated) to maintain the temperature of the house at a desired temperature set by a thermostat control within the house. As a consequence of its load-following capabilities, the RPC heat pump will operate more or less continuously. This is in sharp contrast to the on-off cycling type of operating characteristic of most currently available (non-modulating) heat pumps. It has been determined, as a result of the above briefly described experience, that the modulating mode type of heat pump operation is more energy efficient than on-off cycling and can result in significant kilowatt-hour savings (operating cost savings) for the home owner having an RPC heat pump installation.
The RPC heat pump modulating capability is attained by means of a unique linear reciprocating motor-driven resonant free-piston compressor. To obtain the continuously variable output capability, the displacement (stroke) of the RPC is changed or varied, and hereinafter will be referred to as stroke control or stroke modulation.
Studies have indicated that by providing a control capability for continuously adjusting piston stroke (stroke modulation), RPC-equipped heat pumps should yield significant energy savings and fast pay back of installed cost premiums. Compared to the best residential heat pumps concurrently available (those heat pumps with constant-displacement compression operating in an on-off cycling mode), the advantages of a continuously variable modulation RPC heat pump predictably include a 19%-34% reduction in annual operating costs for the owner; depending upon geographic location, a 2.2 years or less pay back time on installed cost premiums, again depending upon geographic location; higher inherent reliability resulting from RPC tolerance to liquid refrigerant flood back and greatly reduced number of heat pump on-off and defrost cycles as a consequence of the continuous modulation capability, and; lower maintenance and repair costs due to reduced stress on components as a consequence of the stroke modulation control capability in constrast to the higher stress on-off cycling used in conventional heat pumps.
In order to take full advantage of the stroke-modulation capability inherent in the RPC as described in the above-listed patents, it was desirable to provide a better means for controlling (modulating) the resonant free-piston stroke of the compressor over a relatively wide range of outdoor temperatures. For this purpose, the present invention was devised.